Hydraulic actuators have widely been used in the field of construction machine because the component can be reduced in size and weight irrespective of its output power. The hydraulic actuator has lower energy efficiency than the electric actuator; therefore, mounting the electric actuator has recently been studied. In particular, an actuator that drivingly swings the upperstructure of a construction machine relative to the undercarriage is frequently used and is of a rotary type. Therefore, it is effective to replace the hydraulic actuator with an electric actuator.
A swing drive system using an electric actuator was experimentally manufactured and researched. However, it was revealed that a problem with safety is likely to occur if the swing drive system using the electric actuator is operated in the same manner as the swing drive system using the hydraulic actuator because of a difference in output characteristic between the electric actuator and the hydraulic actuator. Concretely, the following was revealed. The swing drive system using the electric actuator is controlled by a speed command or torque command. When swing is operatively started and then operatively stopped, the swing drive system using the electric actuator controlled by a torque command is not stopped in the same way as the swing drive system using the hydraulic actuator. Thus, the travel distance until the stoppage is great. If so, a front attachment or the like connected to the upperstructure is liable to collide with an obstacle present in the swing direction, lowering safety. On the other hand, the swing drive system using the electric actuator controlled by a speed command is rapidly stopped as compared with the swing drive system using the hydraulic actuator when swing is operatively started and then operatively stopped. If an arm is rapidly stopped, then heavy goods such as stones and rocks put in a bucket may be scattered in some cases, lowering safety.
There is known a swing drive system for a construction machine which controls torque characteristics of an electric actuator during starting and during braking by resembling the hydraulic actuator in torque characteristics during those. In addition, this swing drive system uses the electric motor characteristic of an electric motor during swing acceleration and uses the generator characteristics of the electric motor during swing deceleration. In this way, the swing drive system uses torque characteristics different from each other between during swing acceleration and during swing deceleration (refer to e.g. patent document 1).    Patent Document 1: JP-A-2001-11897